Maximizing MRR with Tools for High

Advanced cutting tools can maximize metal removal rates (MRR) when machining even the most difficult-to-machine materials. Powered by the latest CAM programs, these machining strategies are known variously as high-speed, high-efficiency, optimized roughing and also by proprietary brand names like Mastercam's Dynamic Milling. Tools such as multi-flute, solid-carbide tools benefit from the latest advanced technologies in machine look-ahead, high-speed spindles, coatings and geometries.

Here's how leading tooling manufacturers are helping customers put these tools to work in machining titanium, nickel-based alloys, superalloys, Inconel and stainless steel.

Removing metal is important, and doing it fast enough to make money is more important. To capitalize on the latest machining strategies for milling difficult-to-machine materials, Iscar Metals Inc., Arlington, Texas, continues to add to its lines of multi-flute, solid-carbide end mills, according to Bryan Stusak, national product manager–milling. Iscar has designed solid-carbide end mills specifically for milling strategies, including high-speed milling, high-efficiency milling, optimized roughing and proprietary CAM strategies like Mastercam's Dynamic Milling.

"All four of those strategies are essentially the same," said Stusak. "We have developed multi-flute tools, and specifically a seven-flute tool with chip splitting technology to allow very light widths of cut depending on the length of the flute on the end mill. These strategies actively manage all four of the attributes in the CAM systems—including the radial width of cut, the arc of contact, chip thickness and the feed rate—for optimized performance," he said.

Chip-splitting technology reduces the radial tool pressure encountered with long lengths of cut and helps break up the chips, producing more manageable chips for the operator or the chip pan or conveyor to remove, Stusak explained. "The key to machining difficult-to-machine materials is the radial engagement," he said. "You want to minimize the width of cut or arc of contact to beat the heat." By minimizing the width of cut, not as much heat transfers into the tool because of the limited amount of cut time on the end mill.

There are other advantages. "By minimizing the width of cut you can elevate the surface footage on most alloys, with the exception of nickel-based alloys," said Stusak. "You can't elevate cutting speed that much because it is impossible to eliminate the heat in the cut, but for Ti6Al4V we have case studies where we have machined up to 400 sfm at 4 percent radial engagement with these tools."

Understanding the composition of these materials is key to understanding the limitations with cutting speed. "Workpiece hardness and material composition have a huge bearing on machinability," he explained. "Nickel-based, cobalt-based, and ferrous-based superalloys have certain alloying elements in them that won't allow elevating the sfm because you can't eliminate the heat in the cut no matter what you do with the width of cut or cutting speed. [Cutting speeds have] to stay between 80 to 110 sfm depending on the hardness of the material."

It is different with PH stainless, some duplex stainless steels, and titanium alloys where speed can be increased to get more productivity out of the tool. "Duplex stainless steels that have a lot of nickel and chrome content machine more like Inconel materials because of the high nickel content. So, it's essential in machining high-temperature alloys to understand the alloying elements in them," he said.

Stusak emphasized the benefits of these machining strategies by explaining that the basic principle of metal cutting is forming a chip properly in relationship to edge geometry so that you are shearing the material, not plowing it. Both roughing and finishing benefit from optimized machining strategies, but especially roughing, where machining time can be greatly reduced.

"Finishing is typically done with a 45o helix end mill for hardened materials up to 65 HRC because the higher helix angle shears the material more effectively," he said. "End mills with a 60o helix angle are used on nonferrous materials like aluminum and even high-nickel-content alloys in finishing applications. In general, a variable pitch end mill with a 35 to 38o helix angle is the most common we see in the industry because it has a good balance of edge strength and core diameter, and it's a little bit more up sharp in the cut where it slices through the material more effectively vs a 30o helix end mill."

Iscar's families of end mills for high-speed milling include the following:

The ECP-H7-CF multi-flute (seven flutes) end mill has a hard substrate, IC902 ultra-fine carbide grade with 9 percent cobalt, and is TiAlN PVD coated. It is suitable for machining various materials, including hard steel and cast iron, at high cutting speeds, according to Iscar.

The ECY-S5 end mill with five flutes features a general-purpose substrate and AlTiCrSiN coating (IC608) for shoulder or full slot high-speed milling or trochoidal or peel milling. Its primary application is stainless but it can also be used to machine nickel-based, high-temperature alloys.

The ECI-H4S-CFE end mill is a short, four-flute design with different helixes (35o and 37o) and variable pitch for chatter dampening. It can be used for high MRR roughing and finishing, with full slot milling up to 1×D. It is also available with the new AlTiCrSiN IC608 coating for machining at elevated temperatures.

The ECKI-H4R-CF four-flute end mill features corner radii for aerospace applications and either of two coatings, IC300 TiCN or IC900 AlTiN. It offers variable pitch and variable helix and a special edge prep for machining titanium.

With high-temperature-resistant, nickel-based alloys being used more commonly by its customers, Seco Tools LLC, Troy, Mich., is focused on maximizing metal removal rates using high-speed, high-efficiency optimized roughing strategies, according to Jay Ball, solid-carbide product manager.

"Processing these materials with conventional machining processes tends to work harden them," he explained. "Using high-efficiency milling and optimized roughing, there is a lot less heat generated because you are taking lighter radial stepovers and depths of cut (DOC), but not putting a lot of heat into the workpiece," he said. "Where the typical solid-carbide end mill used for roughing and finishing typically had four and five flutes, with high-efficiency milling now taking over the industry we have added six-, seven- and nine-flute tools."

The advantage of multi-flute end mills is that operators can take higher feed rates because of the reduction in DOC and stepover with high-temperature, heat-resistant materials. "These metals don't like to be machined in the conventional way with large DOCs and large radial stepover and slow feed rates," said Ball. "Multi-flute tools allow increased MRRs without work hardening because you can run faster feed rates and lighter radial stepovers with more teeth."

He pointed out that while getting material roughed out is difficult and can cause multiple problems, optimized roughing with 6-10 percent maximum radial stepovers is effective on heat-resistant superalloys (HRSA) and titanium. "And you can use these same tools to then finish a lot of these parts as well so you are using more traditional side mill finishing," he said.

Seco Tools has developed specific geometries, coatings, carbide substrates and edge preps for these difficult-to-machine materials. The company's latest development in coatings is its patented HXT silicon-based coating for higher thermal resistance and abrasion resistance. "What we have found is that these same tools can be used to cut easier-to-machine metals such as tool steels, stainless steels and cast iron. So we’re now able to use these high-efficiency milling strategies to increase tool life and productivity on a broader range of easier-to-machine materials," said Ball.

He added: "We have started to play a lot more with variable indexes and [helixes] in multi-flute cutting tools because of their potential for more cutting pressure due to increased tool contact with the workpiece. However, it's necessary to change [helixes], rakes and indexes to vary the geometry in such a way that it breaks up chatter and harmonics and still retains the tool's ability to cut efficiently."

These optimized high-speed and high-efficiency machining strategies are the wave of the future. And they are here today. According to Ball, 80-90 percent of CAM software suppliers have some sort of optimized milling strategy for roughing and 80-90 percent of the major cutting tool manufacturers have some sort of multi-flute products for these strategies.

The objective of both high-speed and high-efficiency machining strategies is to improve MRR, according to Yair Bruhis, global product and application manager for YG-1 Tool Co., Vernon Hills, Illinois. High-efficiency machining increases cutting by limiting air cutting time. "Because the two machining strategies are so effective, people want to switch everything towards them," said Bruhis. "But it all depends on the part and the machining parameters. Sometimes, I can look at the part and state that it can't be machined with high-efficiency strategies because of the shape and complexity of the part, or the machine's capabilities, or the part features and programming, among other factors.

"I talk to a lot of people in aerospace and the trend has changed in the last 10 or 15 years," Bruhis continued. "It's not the cost of the tool any more. Customers want to know the real cost of metal removal. There are a lot of cases where I meet with engineers or programmers and they clearly voice that they do not care about the price of the tool. Cycle time and tool life are the most important considerations."

He also noted that the trend in titanium alloys and exotics machining in the last four or five years is toward high-speed machining for medium to large parts because the cost of removing titanium or Inconel is much higher than that of aluminum or steel.

"In evaluating machining for large aerospace parts, for example, while I’m not a programmer, in most cases I can look at the program and tell what ought to be changed," said Bruhis. "In the last few years between traveling and working all over the world, if I can't review the program, I have my customer send a video of the simulation and hold an online meeting to discuss possible program modifications. Through Skype interactions, I do simulations and alter programs constantly."

YG-1 has developed standard tools specifically for high-speed machining of titanium, but about 30 percent of its tools for this application are still custom made, with special lengths and corner radii. "One of the trends with high-speed machining is the increased number of flutes needed to take light cuts and run very fast," he said. "The trend of the last five years is for five, six, seven and nine flutes," he said. The advantage is longer tool life and better heat and chip control as well as machining performance.

"When major OEMs call me in, it's generally to improve tool life, the process, or both," Bruhis continued. "It could be a new project with them facing a serious issue. It might be a problem with part quality, or cycle time or delivering parts in time or total cost, but it's almost never because of the cost of the tool since YG-1 offers a very attractive performance-to-cost package."

Bruhis described how he evaluates and determines an approach to a titanium machining project. "I typically inquire first about the machine capability, whether three-, four-, or five-axis, vertical or horizontal, fixturing and tooling," he said. He added that in the majority of cases, specific end mills are selected based on axial or radial cut, speeds and feeds, and programming for high-speed and high-efficiency machining.

Cutter paths vary and can include profiling, slotting, and pocketing. Workpieces can vary in complexity and size as well. YG-1 has tools for specific materials like titanium, Inconel or aluminum as well as general-purpose tools for smaller shops and multiple applications.

"We determine the process and program and run it within a range of speeds and feeds and estimate a cycle time," Bruhis said. "Once the customer has a chance to run the program that we have set, we then can get feedback with real machining time results and, if the cycle time is too long and the cost is not in line with expected results, we make the required adjustments."

Like other companies contributing to this article, Horn USA Inc., Franklin, Tenn., stresses both the importance of multi-flute tool design and customer collaboration for tooling success. "I would describe us as an engineering-driven company that approaches tooling solutions for its customers with finesse," said Edwin Tonne, training and technical specialist. Horn, which is well known for its grooving and cut-off turning tools, offers a broad line of products, including solid-carbide end mills, drills, and indexable milling cutters, as well as its turning products. More than 40 percent of its cutting tools are specials. Horn has developed multi-flute end mills used to machine titanium, Inconel, stainless and other high-temperature-resistant metals using high-speed and high-efficiency machining strategies to achieve the highest MRR.

The following is a consensus report of an interview held with Tonne, Eric Carbone, application and sales engineer; John Kollenbroich, head of product management; and Jeff Shope, application and sales engineer.

Not every part is a good candidate for high-speed machining. The choice of strategy is a function of the part geometry and size. Some of the testing that's being seen has been calling for machining Inconel, titanium, and stainless with light depths of cut, high speeds and low radial engagement and feed rates.

If it's a very shallow "depth of part," the machinist will not get the economy of the end mill and high speed and will experience a lot of excess vibration. The reason is if a shop runs a shallow axial depth of cut, it reduces MRR and the operation may not be as efficient as other methods with larger radial and shallower axial stepovers.

These machining strategies require more just than the right carbide grade, insert and geometry—the way of approaching the material is also critical. The goal of high-efficiency machining is to reduce the width of cut and increase the length of cut to reduce cutting forces, which allows faster machining. Sometimes it is quicker that way, and sometimes it is quicker using traditional high-feed cutters. Many times with dynamic machining there can be much wasted movement. Applying it depends on the application and complexity of the features, such as pocketing, that are involved.

It's important to have the right CAM software to avoid wasted rapid travel movement, which increases cycle time. There are times when it is better to take a more conventional cutting pass. One example is when the width of cut is short with, say, a 0.5" (12.7-mm) end mill with the intention of cutting a length of the part that's 0.5" long and the process needs to remove 0.3" (7.62 mm) of material. In this example, Horn recommends taking all the material off in one or two passes instead of 30 passes. To be efficient, the tool must stay on the part and limit time-wasting retraction.

Besides the component, programming strategy and software play into this as well. If a shop is performing high-efficiency or high-speed milling, it must have the horsepower and torque required to drive the tool. If it runs the wrong software, there will be a lot of costly, wasted moves.

Horn's solid-carbide tools with seven or nine flutes with large DOCs and 10-15 percent stepover—as a rule of thumb to start with—help with these strategies, but the machine tool must have the required acceleration and deceleration. An older machine with rapids of 600 ipm will not be sufficient. Similarly, the look-ahead that newer machines have is also required.

Horn's DSFT end mills—part of the DS line of high-DOC, low-radial-engagement tools—are designed for trochoidal machining. To be effective, DS tools require a solid machine spindle with close runout and a capable controller for programming. CAD programs are available to create simulation of machining time estimates to decide whether traditional end milling or high-speed machining is best. In addition, there are a number of software tools available to evaluate the economics of these tooling decisions.

The highest MRR possible in high-speed machining with multi-flute tools occurs when the process engages the full flute length of the tool. The more flutes, the larger the core diameter for rigidity. Typically, the first thing to look at when considering high-speed machining is the size of the part and flute length to decide the diameter of the tool, according to Horn. An inch of actual flute length might be handled by a 3/8" (9.5-mm) diameter tool, and two inches of actual flute length by a 5/8" (15.8-mm) diameter tool.

The goal is to maximize flute length because that's what will provide the best MRR in combination with 5 and 10 percent stepovers. Another way to determine tool selection is to decide whether to simply switch to high-feed milling and ramp in with a conventional end mill and rip the stock out.

Cycle times for five-axis machining of molds, blades and other complex aerospace and medical parts can be reduced up to 90 percent with Circle Segment solid-carbide end mills, according to Emuge Corp, West Boylston, Mass. While manufacturers performing high-speed machining may be familiar with using traditional ball nose end mills to make small stepover passes, Circle Segment end mills use high stepover passes up to 10 times greater than ball nose end mills to cut out large areas of material, maximizing efficiency and minimizing cusp height.

According to the company, time and cost savings and increased part quality result. Tool life is increased due to shorter toolpaths. Tolerance deviations due to heat warping at the tool are minimized, and axial deviations of the machine are smoothed, offering a higher quality surface finish in a shorter time frame. Circle Segment end mills feature unique forms with large radii in cutting areas of the mills, allowing a larger axial DOC during prefinishing and finishing operations.

The end mills are available in four geometries: barrel-shaped, oval form, taper form and lens shape. Oval and taper form mills are suited for curved shapes such as blades or straight-walled pockets, freely engaging more of the cutting edge. Barrel design mills provide effective flank milling to the sides of spiral grooves and similar applications, according to Emuge. Lens-shaped mills excel in narrow channels or in lands on molds. Specific CAM systems software, such as Mastercam and hyperMILL, are required to support and compute the geometries.

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